Zeta potential electrophoretic light-scattering

Thompson, R.G., Practical zeta potential determination using electrophoretic light scattering, Am. Lab., 24, 48, 1992. 

Dynamic light scattering (DLS) techniques measure the fluctuations in the scattered light intensity caused by the random Brownian motion of the dispersed particles. The use of a theoretical model of particle Brownian motion enables us to extract particle size from DLS data. Other dynamic light scattering techniques such as electrophoretic light scattering (ELS) study collective particle motions. Theoretical interpretation of ELS data leads to other particle properties such as electrophoretic mobility fi and zeta potential f. These techniques will be discussed in more detail in subsequent sections. 

As an optical detection system, electrophoretic light scattering has been frequently used in recent years [4], In this case, the collected signal of the particles movement shifted to higher or lower frequencies depending on their charge. The frequencies are then converted to electrophoretic mobilities, velocities, and finally zeta potentials. 

The BaS04 suspension was then titrated stepwise with polyelectrolyte and allowed to equilibrate for about 30 minutes after each addition. The zeta potential of the resulting suspension was then determined by Doppler electrophoretic light scattering analysis using a Coulter Electronics DELSA 440. BaS04 suspensions were titrated to a toti polyelectrolyte concentration of 100 mg polyelectrolyte/m BaS04. 

An instmmental alternative to microelectrophoresis is electrophoretic light scattering (ELS). The light scattering at migrating particles leads to phase shift (Doppler effect), which can be detected by a heterodyne DLS set-up (i.e. reference-beating with frequency shift). The method yields an intensity weighted distribution of the zeta-potential. 

The electrical charge of emulsion droplets was determined via zeta-potential measurement by electrophoretic light scattering. The oil droplet size of the emulsions was analysed by static light scattering after dilution of the emulsion to the required optical density or, in the case of spray-dried particles, after dissolution of an aliquot of the microcapsules. Microencapsulation efficiency was calculated from total oil content in the formulation and the gravimetric determination of the oil extracted from the microcapsules with petrol ether [62]. 

The present chapter is dedicated to how the zeta potential of microparticles is characterized. It is divided into five sections as follows Section 6.2 defines and describes zeta potential and introduces the most popular way to measure zeta potential, i.e., the electrophoretic mobility measurement. Section 6.3 describes the modem technology used to measure electrophoretic mobility -electrophoretic light scattering (ELS), including the optics and the sample cell followed by the data analysis of ELS experiment in Section 6.4. Section 6.5 introduces a new technique in ELS, known as phase analysis light scattering (PALS) that can be used to measure extremely small mobilities in non-polar media. 

Recent development of laser-based instrumentation for electrophoretic mobility determination has made it possible to determine zeta potential of particles suspended in liquid media for systems that were difficult or impossible to study by classical techniques. These instruments measure electrophoretic mobilities by making direct velocity measurements of particles moving in an applied electric field by analyzing the Doppler shift of laser light scattered from the moving particles. Electrophoretic mobilities can then be converted into zeta potentials by use of standard equations 48-50),... 

The electrophoretic motion is either measured microscopically or by light scattering. The former way is called microelectrophoresis and usually employs ultramicroscopes when dealing with colloidal particle systems. The optical instrumentation can be identical to that of DUM, while the software has to be modified because only the displacement in the direction of the electric field is relevant. The method yields a number weighted distribution of zeta-potentials. Similar to DUM, a sufficiently large number of trajectories has to be evaluated in order to keep the statistical uncertainty within an acceptable level. Moreover, the method may be insensitive to weak scatterers within a polydisperse colloidal suspension. 

The final method to be mentioned is Zeta potential (may also be referred to as an electrophoretic method). In this method, the diluted emulsion is placed in a measurement ceU, and a static elecflic field is applied. Emulsion droplets carrying an electrostatic charge wiU move in the field at a velocity that is a function of several factors, including viscosity of the emulsion, net charge on the particle, and particle size. The velocity of movement can be determined using a light scattering method... 

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